Measurement of small linear displacements without interfering with the displacement of the object being displaced generally involves the use of costly devices, precision alignments, and frequent realignments and recalibrations. Optical devices are used because they do not interfere. These include dual-laser range finders, single- or dual-laser radar systems, and interferometers. The latter devices may utilize optical gratings for detecting fringe patterns. Non-optical systems include capacitance devices which are difficult to linearize.
Optical positioning systems are utilized for precision alignment of semiconductor patterns with masking plates using laser light. For example, in U.S. Pat. No. 4,377,028 (Imahashi) reflected light from a pattern on a wafer is focused through a mask with the same pattern, and the transmitted light is detected to establish alignment. U.S. Pat. No. 4,265,542 involves using interference and diffraction effects produced by coherent light impinging upon or passing through repetitive patterns on a photomask and a semiconductor surface.